The invention relates generally to Magnetic Resonance Imaging (MRI) systems and more specifically to a method and apparatus for parallel imaging in the frequency-encoding direction using MRI systems.
MRI is a well-known imaging technique. A conventional MRI device establishes a homogenous magnetic field, for example, along an axis of a person's body that is to undergo MRI. This homogeneous magnetic field conditions the interior of the person's body for imaging by aligning the nuclear spins (in atoms and molecules forming the body tissue) along the axis of the magnetic field. If the orientation of the nuclear spins is perturbed out of alignment with the magnetic field, the nuclei attempt to realign their spins with the axis of the magnetic field. Perturbation of the orientation of nuclear spins may be caused by application of radio frequency (RF) pulses tuned to the armor frequency. During the realignment process, the nuclei precess about the axis of the magnetic field and emit electromagnetic signals that may be detected by one or more RF detector coils placed on or about the person.
The frequency of the magnetic resonance (MR) signal emitted by a given processing nucleus depends on the strength of the magnetic field at the nucleus' location. As is well known in the art, it is possible to distinguish signals originating from different locations within the person's body by applying a gradient to the magnetic field across the person's body. For the sake of convenience, direction of this field gradient may be referred to as the left-to-right direction. Signals of a particular frequency acquired during application of the field gradient may be assumed to originate at a given position within the field gradient, and hence at a given left-to-right position within the person's body. The application of such a field gradient is also referred to as frequency encoding.
However, the application of a field gradient does not allow for two-dimensional spatial encoding, since all nuclei at a given left-to-right position experience the same field strength, and hence emit signals of the same frequency. Accordingly, the application of a frequency-encoding gradient, by itself, does not make it possible to discern signals originating from the top versus signals originating from the bottom of the person at a given left-to-right position. Spatial encoding has been found to be possible in this second direction by application of gradients of varied strength in a perpendicular direction prior to acquisition of the signal, to thereby twist the phase of the nuclear spins by varied amounts. The application of such additional gradients is referred to as phase encoding.
Frequency-encoded data sensed by the RF detector coils after a phase encoding step are stored as a line of data in a data matrix known as the k-space matrix. Multiple phase encoding steps are performed in order to fill the multiple lines of the k-space matrix. An image may be generated from this matrix by performing a two-dimensional Fourier transformation of the matrix to convert this frequency information to spatial information representing the distribution of nuclear spins or density of nuclei of the image material.
Alternatively, spatial encoding can be performed in three dimensions by applying phase-encoding gradients in two orthogonal directions, followed by a frequency-encoding gradient in the third orthogonal direction, with signals acquired during the frequency-encoding gradient, in order to generate a three-dimensional matrix of k-space data. Three-dimensional Fourier transformation of the matrix will then convert this frequency information to spatial information representing the distribution of nuclear spins or density of nuclei within a volume of the image material.
Imaging time is largely a factor of desired signal-to-noise ratio (SNR) and the speed with which the MRI device can fill the k-space matrix. In conventional MRI, the k-space matrix is filled one line at a time. Although many improvements have been made in this general area, the speed with which the k-space matrix may be filled is limited. To overcome these inherent limits, several techniques have been developed to effectively simultaneously acquire multiple lines of data for each application of a magnetic field gradient. These techniques, which may collectively be characterized as “parallel imaging techniques”, use spatial information from arrays of RF detector coils to substitute for the encoding which would otherwise have to be obtained in a sequential fashion using field gradients and RF pulses. The use of multiple effective detectors has been shown to multiply imaging speed, without increasing gradient switching rates or RF power deposition.
Parallel imaging techniques fall into one of two categories. They can fill in the omitted k-space lines prior to Fourier transformation, by constructing a weighted combination of neighboring lines acquired by the different RF detector coils. Or, they can first Fourier transform the limited k-space data to produce an aliased image from each coil, and then unfold the aliased signals by a linear transformation of the superimposed pixel values. In either case, the reconstructed images tend to suffer from incomplete removal of aliasing artifacts, especially for large speedup factors. In images corrupted by aliasing, the edges of the image are wrapped into the center of the image.
Two such parallel imaging techniques that have recently been developed and applied to in vivo imaging are SENSE (SENSitivity Encoding) and SMASH (simultaneous acquisition of spatial harmonics). Both techniques include the parallel use of a plurality of separate receiving elements, with each element having a different respective sensitivity profile, and combination of the respective spin resonance signals detected enables a reduction of the acquisition time required for an image (in comparison with conventional Fourier image reconstruction) by a factor which in the most favorable case equals the number of the receiving members used (see Pruessmann et al., Magnetic Resonance in Medicine Vol. 42, p.952-962, 1999).
For pulse sequences that execute a rectilinear trajectory in k space, parallel imaging techniques invariably reduce the number of phase encoding steps in order to reduce imaging time, and then use array sensitivity information to make up for the loss of spatial information. For some pulse sequences, such as single-shot fast spin echo, substantial reductions in imaging time could also be realized if the number of points in the frequency-encoding direction could be reduced without sacrificing spatial resolution or field of view (FOV). Receiver filtering cuts off the signals outside the readout band, unlike in the phase-encoding direction, where such signals are aliased back into the FOV, where they can be unwrapped by parallel image reconstruction.
Multi-coil MR imaging utilizes a greater number of receiving channels. Raising the number of channels in multi-coil imaging beyond the typical 4 to 8 can potentially yield a wide variety of benefits, including improved signal-to-noise-ratio (SNR), larger field of views (FOV) and/or faster imaging speeds. These benefits may have important applications in overcoming cardiac, respiratory, and peristaltic motion during body imaging, and for delivering and monitoring therapy under MRI guidance. A method and system are needed for performing accelerated parallel imaging using MRI.